feat: add support for using existing parameter group

[bkero]

what

Allows an existing parameter group to be specified. This is useful for cases where you simply want to use an existing parameter group (such as default.redis7), or when a parameter group is handled in other places within Terraform.

why

We have some clusters which are using existing parameter groups who's management is out of scope for this module. We would like to use them with clusters deployed using this module.

references

[y3ti]

Duplicated #208

[bkero]

Duplicated #208

Hey there. I just read #208 and it that it has a similar goal, although still enforces creation of a parameter group. The impetus for this pull request is to allow an already-existing parameter group to be specified. If you look at the count metaparameter of the parameter_group resource, you'll see that I'm preventing the creation if a name is specified.

[y3ti]

@bkero ach ok, you are right. Thank you for explaining it once more :)

What if we have a new variable create_parameter_group? If the variable's value is set to false then the parameter group won't be created. If set to true then the new parameter group will be created with the name set to ${parameter_group_name}. This approach allows us to address both scenarios effectively.

[bkero]

@bkero ach ok, you are right. Thank you for explaining it once more :)

What if we have a new variable create_parameter_group? If the variable's value is set to false then the parameter group won't be created. If set to true then the new parameter group will be created with the name set to ${parameter_group_name}. This approach allows us to address both scenarios effectively.

Yes I would find that perfectly satisfactory. It would allow me to use the default default.redis7 parameter group.

I can create a follow-up PR once #208 merges.

[y3ti]

@bkero I added the new variable create_parameter_group to my PR: #208 (comment)